Conventionally, rechargeable electric appliances, such as an electric toothbrush and an electric shaver, that are used mainly in wet areas have adopted contactless chargers to avoid exposure of connection electrodes provided for connection between the chargers and the electric appliances. Due to safety concerns, the use of such a contactless charger has recently expanded to a domestic game machine, a cordless phone, a mobile phone, and others. Generally speaking, there is a one-to-one correspondence between chargers and the electric appliances, and different electric appliances require dedicated chargers.
Incidentally, portable terminal devices, such as a mobile phone and a smartphone, are facing the problem of how to secure the power sources while their demands are dramatically increasing. These portable terminal devices also secure the power sources by using dedicated chargers or AC adaptors. However, assuming that one wishes to secure the power sources when operating the portable terminals away from home without carrying dedicated chargers or AC adaptors, it is necessary to prepare different chargers or adaptors for different portable terminal devices. This requires a great number of chargers or adaptors and is unfeasible. Adopting contactless charging method provides an advantageous effect of allowing flexible power source connection that has nothing to do with specifications of power source connection terminals. This has driven the need for normalization and standardization of charging methods between charging devices and portable terminal devices.
As an example, the Wireless Power Consortium (WPC) has released the standard contactless charging method called Qi (pronounced “chee”) targeted mainly for portable terminal apparatuses, thus enabling charging between any Qi-compliant chargers and portable terminal devices.
The contactless charging method, including the Qi standard, involves power transfer through inductive coupling or magnetic resonance between a primary antenna included in a charging device and a secondary antenna included in a power-receiving device.
A contactless charging system (hereinafter, may be called a contactless power system) as such includes resonant circuits configured by connecting a resonant capacitor to each of the primary antenna and the secondary antenna for the purpose of contactless power transfer and data communication between the charging device and the power-receiving device. Regulating the resonant frequency of the resonant circuit on the primary side and that on the secondary side allows stable and efficient power transfer and data communication between the charging device and the power-receiving device.
In this regard, the inductance L of each antenna and the capacitance C of each resonant capacitor are subject to several variable factors and are not always predictable. For example, the characteristics of the inductance L change depending on variation in characteristics of a magnetic core used in the antenna and on an ambient temperature. The capacitance C of the resonant capacitor also changes depending on initial variation, temperature characteristics, and voltage dependency. Furthermore, a mutual inductance M of the primary and the secondary antenna changes depending on a clearance and relative positioning between the primary and the secondary antenna, and since the charging device is physically distant from the power-receiving device, it is difficult to maintain fixed relative positions with respect to each other.
When the resonant frequency is shifted due to the aforementioned various factors, power transfer efficiency is deteriorated, and the problem of heating manifests itself. This prevents miniaturization and reduction in power consumption of the device. A significant deterioration in transfer efficiency might also cause even a system problem such as prolonged charging time and a timeout in charging.
Accordingly, the big challenge is to regulate the resonant frequency on each of the primary and the secondary side to an optimal value.